This invention relates in general to improved, thickened, fluid compositions to remove ice, frost, and snow from surfaces and/or to prevent ice from forming on surfaces. More specifically, in certain embodiments, this invention relates to formulation of environmentally-friendly, non-Newtonian fluids, primarily for aircraft deicing/anti-icing, as required per Society of Automotive Engineers Aerospace Material Specification (SAE/AMS) 1428. However, other applications of deicing/anti-icing, such as wind-turbine blades and third-rail where the rail is for making electrical contact, are also possible.
Several types of aircraft deicing/anti-icing fluids (ADFs/AAFs) are used to remove deposits of ice, frost and snow from aerodynamically-critical surfaces before an aircraft can safely lift off the runway. Additionally, the film of such a liquid, left on the aircraft, provides some protection from refreezing of water due to freezing precipitation. The anti-icing property of these fluids can be quantified by the Water Spray Endurance Test (WSET), which is fully described in SAE/AMS 1428 Specification. Four different types of ADF/AAFs, namely Types I, II, III, or IV, can be qualified for use on an aircraft, depending on how the fluid is used and the anti-icing protection achieved. The anti-icing protection requirement for Type I fluids is only 3 minutes, with Types II and IV being 30 and 80 minutes, respectively. To achieve the much longer WSET times for Types II and IV fluids, the fluids are thickened and these exhibit non-Newtonian (pseudoplastic) behaviors, also referred to as shear-thinning behavior. The shear thinning behavior allows for maximum anti-icing protection due to the uniform coverage by a high viscosity fluid when the aircraft is stationary (zero shear). This fluid greatly thins out during an aircraft take-off roll as the shear rate rapidly increases, which allows the aircraft to shed the majority of the fluid, thus restoring the aerodynamics of the airfoil.
The primary difference between Type II and Type IV fluids is that a Type II can be used both for deicing and anti-icing while Type IV is used just for anti-icing after a deicing step is completed. In either case, such fluids typically contain a mixture of a glycol, such as ethylene glycol (EG) or diethylene glycol (DEG) or propylene glycol (PG) with water, the glycol plus water typically adding up to more than 95% by weight (wt %). These fluids also contain additives such as thickeners, surfactants, anti-foamers, corrosion inhibitors, anti-precipitants, and dyes to meet the specifications. All of these chemicals are of potential concern to airports as they are required to obtain storm water discharge permits under the applicable environmental protection agencies. The concerns are: (a) high levels of oxygen demand due to natural biodegradation of glycols; (b) mammalian toxicity of some glycols, such as EG (which is also a hazardous air pollutant) and DEG; (c) toxicity of degradation products of commonly-used surfactants; (d) toxicity and non-biodegradability of commonly used corrosion inhibitors and other additives.
The vast majority of Type II and IV fluids use PG as a freeze-point depressant but it has a high chemical oxygen demand (COD) and biological oxygen demand (BOD). The thickened fluids typically use alkylphenol ethoxylate (APE) surfactants, the biodegradation products of which have been shown to be endocrine disruptors, and as such these are banned in Europe and are under EPA scrutiny in the U.S. A number of fluids also use benzyltriazole or tolytriazole corrosion inhibitors, which are toxic and non-biodegradable and thus persist in the environment.
The thickened fluids of prior art typically use large molecules (polymers) that typically thicken by molecular entanglement or gelation due to pH adjustment. These polymers often have performance deficiencies as they leave gel-forming residues on aircraft surfaces. The quantity of residue is typically proportional to the amount of thickener (polymer) used as that is the primary controller of viscosity and rheology. Additionally, these thickeners typically have relatively flat viscosity vs. temperature curve in the 20° C. to −30° C. range. If a certain, high viscosity is targeted for the 0 to −10° C., which is the majority of operating range, one gets undesirably high viscosities at 20° C. which makes fluid preparation and handling harder as well as at temperature below −10° C. which makes aerodynamic performance poorer and the gel residue problem worse.
Various patents are known that describe some of the approaches, partial solutions, and inherent problems for addressing the requirements for aircraft anti-icing. However, the thickened aircraft fluids described in the patents have functional and/or environmental deficiencies. The functionally superior formulations rely on use of components that were recently recognized as environmentally unfriendly and are in various phases of being banned. A high performance, environmentally friendly, thickened aircraft fluid is therefore desired. Such fluids may also be useful for other applications, such as (a) deicing/anti-icing of wind-turbine blades and (b) stationary surfaces such as the third rail for electric trains.